One of the most important characteristics of pumps is suction capacity expressed by the cavitation critical speed coefficient: ##EQU1## where n--drive shaft rotational speed, in revolutions per minute;
Q--volumetric rate of flow of the liquid being pumped, i.e. pump output, in cubic meters per second; PA1 .DELTA.h--net positive suction head, in meters. PA1 D.sub.o --inside diameter of the pump housing at the entry to the suction wheel; PA1 D.sub.1 --inside diameter of the pump housing at the entry to the impeller wheel; PA1 K.sub.1 --dimensionless coefficient of 0.17 to 0.13; PA1 C.sub.k --predetermined cavitation critical speed coefficient of 5,000 to 11,000. PA1 .beta..sub.o --vane tip setting angle of the suction wheel at the entry thereto; PA1 .DELTA.--radial clearance at the entry to the suction wheel; PA1 D.sub.1 --inside diameter of the pump housing at the entry to the impeller wheel.
The greater the coefficient C.sub.k, the greater the pump suction capacity.
The rotational speed of the pump drive shaft determines the size and mass of the pump, whereas the pump output and the suction capacity determine, respectively, the quantity of the pumps required for the given job and the capital outlay. For example, doubling the pump suction capacity, with unchanged suction head, enables doubling the rotational speed of the pump drive shaft, whereby the pump size and mass can be decreased twice or thrice, making for substantial reduction of pump manufacturing costs. The current trend toward increase in the capacity of single power units calls for further increase in pump output with consequent increase in suction head. In high output pumps, increasing suction head is restrained by cost considerations. On the other hand, increasing pump suction capacity, for example, twice, makes it possible to use one large-output pump instead of four pumps with an equivalent total output and to decrease suction head outlay at least thrice.
Thus, there is a great need in pump engineering for increasing pump suction capacity.
Insufficient suction capacity of a pump causes cavitation with resultant decrease in head and efficiency.
The specific point of the problem is that increase in the suction capacity of a pump is usually accompanied by decrease in the pump efficiency .eta., which causes substantial increase in power consumption. Therefore, as a rule, pumps with high suction capacity have low efficiency, whereas pumps with high efficiency have low suction capactiy.
Known in the art are pumps with high suction capacity (C.sub.k .apprxeq.4,000) ("Kavitatsia v lopastnykh nasosakh", by Stripling, Tr. ASME, Ser. DN 3, 1962/in Russian/).
Such a pump comprises an axial-flow impeller wheel which is mounted on a drive shaft and has a hub with helical vanes attached thereto. The vanes are profiled along the wheel radius according to the expression r.tg.beta.=cost, where r is the current value of the radius of the axial-flow wheel and .beta. is the vane setting angle between the plane normal to the pump drive shaft and the plane tangential to the vanes.
The suction capacity of this pump is increased by virtue of increasing the cross-sectional area of the pump flow duct and decreasing the vane angle, which results in decrease of the wheel entry velocity ratio .phi., i.e. the ratio of the axial velocity C.sub.1 of the liquid flow to the peripheral velocity U.sub.1 of the pump wheel on the outside diameter thereof. Increase in the cross-sectional area of the pump flow duct is achieved by increasing the outside diameter of the pump wheel and by decreasing the hub diameter as much as possible with respect to strength considerations. This solution ensures decrease in the axial component of the liquid flow velocity and provides the minimum drop of static pressure in the liquid flow, whereby the suction capacity of the pump is increased.
However, this pump has a low efficiency (.eta.=0.5) inasmuch as the velocity ratio is low (.phi.&lt;0.1) due to increase in the cross-sectional area of the pump flow duct, decrease in the axial velocity C.sub.1 of the liquid flow, and breakaway nature of flow through the wheel.
Known in the art are vane pumps with the efficiency .eta. as high as 0.75 to 0.9. "Tsentrobezhnye i osevye nasosy" by A. J. Stepanov, "Mashgis" publishers, Moscow, 1960, pp. 141-164/in Russian/).
Such a pump comprises a housing which accommodates an impeller wheel mounted on a drive shaft and having a hub with vanes attached thereto. The developments of the cylindrical sections of said vanes form a cascade of airfoils set at relatively large angles between the airfoil chord and the cascade front, said angles being suitable for an increased velocity ratio (.phi.&gt;0.2).
However, this pump has a substantially low suction capacity (c.sub.k .apprxeq.1,000) in connection with relatively high axial velocities C.sub.1 of the liquid flow due to decrease in the cross-sectional area of the impeller wheel flow duct.
Known in the art are pumps with high suction capacity C.sub.k is as large as 5,200 to 4,200 (See, for example, "Vysokooborotnye lopatochnye nasosy" by B. J. Borovsky, N. S. Ershov, B. V. Ovsyannikov, V. J. Petrov, V. F. Chebaevsky, A. S. Shapiro, "Mashinostroenie" publishers, Moscon, 1975, p. 13, FIG. 5 p. 202/in Russian/) and pumps with a relative suction velocity S.sub.s of 40,000 to 60,000, where S.sub.s =9.19 C.sub.k (see, for example, Barham H. Lee Application of waterjet propulsion to high-performance boats, "Hover Craft and Hydrofoil", 1976, 15, N 9, pp. 33-43). In these pumps, in order to provide high suction capacity, use is made of an axial-flow wheel mounted on a drive shaft together with an impeller wheel. The axial-flow wheel is highly immune to cavitation and develops a head sufficient to provide for cavitation-free operation of the impeller wheel.
In the pumps of the prior art use is made of the following means in order to increase suction capacity:
a worm with a lengthwise variable pitch (USSR Inventor's Certificate no. 154, published in Bull. "Discoveries, Inventions, Industrial Designs and Trademarks", No. 8 of April, 1963, p. 71).
a taper worm mounted in a confuser (British patent No. 1,218,023, published in July 28, 1968, WEIR Pumps L.T.D.).
a worm with a taper hub, variable diameter and blade pitch, and an entry rake;
an upstream, axial-flow, converging wheel with a taper shroud (USSR Inventor's Certificate No. 158493, published in Bull. "Discoveries, Inventions, Industrial Designs and Trademarks", No. 21 of November 1963, p. 76).
a worm in the form of an axially movable helix (USSR Inventor's Certificate No. 542022 published in Bull. "Discoveries, Inventions, Industrial Designs and Trademarks" No. 1 of 1977, p. 151).
an upstream taper wheel with a helical thread on the outer surface (USSR Inventor's Certificate No. 547554, published in Bull. "Discoveries, Inventions, Industrial Designs and Trademarks", No. 7 of 1977, p. 92).
an inlet device installed before a centrifugal wheel and comprising several rows of vanes gradually increasing in diameter;
an upstream, axial-flow wheel the estimated rate of flow through which is three times greater than that through a centrifugal wheel (U.S. Pat. No. 3,384,022, published in May of 1968, Ebara Manufacturing Co LTD. Japan).
a conical hub changing into a radial-flow wheel, which hub mounts several circular rows of round-section pins installed at a varying angle to the axis of rotation (British Pat. No. 1,417,549, published Dec. 10, 1975, Lucas Industries LTD).
an upstream, single- or multiple-start worm or a conical, ribbed head (DE Application No. 2545736, published in Apr. 25, 1977, Blum, Albert).
an upstream, double-stage, axial-flow wheel wherein the vanes of each stage have different diameter and angle of pitch (British Application No. 1,523,893, published Sept. 6, 1978, Nikkiso Co LTD, Japan).
an upstream, axial-flow wheel with a bypass device for recirculating fluid in the zone of a worm (U.S. Pat. No. 3,723,019, published Mar. 27, 1973, Worthigton Corporation).
The pump constructions considered above do not provide for the maximum possible increase in action capacity. Furthermore, while improving some parameters, for example, cavitation characteristics, they impair others, for example, pump efficiency or stability.
Known in the art is a vane pump (U.S. Pat. No. 3,299,821, 103-88, published Jan. 24, 1967, assignor to Sundstrand Corporation, a corporation of Illinois) comprising a housing and two axial-flow wheels, viz. a suction wheel and an impeller wheel, which are mounted on a common drive shaft and installed with a radial clearance in the housing. The suction wheel has a hub with helical vanes attached thereto, the pitch of the vanes increasing in the direction of flow.
The vane pitch on the suction wheel is chosen so as to provide high suction capacity of the pump, whereas the vane pitch on the impeller wheel is chosen so as to provide the required head and increase pump efficiency. The pump operates as follows. The liquid first enters the axial-flow suction wheel. As the flow passes over the vanes, cavitation originates and develops. At the end of the suction wheel the cavitation ceases. After the suction wheel the liquid, which has acquired some energy, enters the axial-flow impeller which creates, in the main, the required head. The pump under consideration provides high suction capacity (C.sub.k =3,000) and an increased efficiency, but these parameters are not at a maximum inasmuch as the radial clearance of the axial-flow wheels and its relation to the wheel geometry are not stipulated.
The technical solutions described above merely disclose the level achieved in the prior art in the endeavor to provide for a pump to have both high suction capacity and high efficiency, which level is, of course, not at the utmost.